1. Field of the Invention
The present invention relates to a memory device and method for manufacturing the same. More particularly, the present invention relates to a single electron memory device having quantum dots between a gate electrode and a single electron storage element and method for manufacturing the same.
2. Description of the Related Art
As the size of a metal-oxide-semiconductor field effect transistor (MOSFET) becomes smaller, problems arise that do not occur in larger devices. Thus, it becomes difficult to further reduce the size of the MOSFET. For example, as the size of the MOSFET device becomes smaller, problems occur such as lowering of threshold voltage caused by a decrease in effective channel length, drain induced barrier lowering (DIBL), punch-through, deterioration of a gate oxide layer, and an increase in leakage current due to hot carriers generated by an increase in the electrical field in the device. These problems complicate further reduction in the size of a MOSFET.
The most significant problem to be solved by continuously scaling the MOSFET to the range of nanometers (nm) is a physical limit. At the physical limit, the number of electrons participating in the operation of the device becomes similar to the number of thermally fluctuating electrons. Thus, proper operation of a miniaturized MOSFET at room temperature is not expected.
To overcome this problem, development of a new device structure, which may replace the current complementary MOSFET (CMOSFET) structure, is required. Recently, a single electron transistor (SET) has been studied as an alternative new device.
Coulomb blockade is a physical principle of the single electron device. In the single electron device, Coulomb blockade is a condition wherein tunneling is blocked under specific conditions. The specific conditions for Coulomb blockade are when the free energy of a total system, consisting of a charging energy and an electrostatic energy for a junction capacitance, increases or decreases when electrons tunnel through a minute size tunnel junction.
The SET is a switching device that controls current flowing through two tunnel junctions between quantum dots by controlling the Coulomb blockade conditions. The Coulomb blockade occurs through a gate potential, which is capacitively coupled to the quantum dots.
Quantum dots, which are coupled to a channel through a tunnel junction, are used as a storage electrode in a single electron memory device. The single electron memory device is a memory device for recognizing a variation in channel current due to charges stored in the quantum dots as information, “0” or “1.”
Unlike in the MOSFET, the effects caused by thermal fluctuation decrease in the single electron device as the device becomes smaller. A property of the device is determined by the capacitance between the elements constituting the device rather than by the structure of the device. Thus, the single electron device is advantageous for device scaling.
Conventional single electron memory devices store electrons in a single quantum dot 14, as shown in FIG. 1 or in a hybrid structure having a high distribution density nano-crystal array 20, as shown in FIG. 2. In both structures, a MOSFET is used as a sense device. In FIGS. 1 and 2, reference numeral 10 denotes a substrate, reference characters S and D denote a source and a drain, and reference characters G and G1 denote a gate lamination.
In FIG. 1, the gate lamination G includes a tunneling oxide layer 12 formed on the substrate 10 between a source S and a drain D, a single quantum dot 14, and a control oxide layer 16 and a gate electrode 18, which cover the single quantum dot 14. In FIG. 2, the gate lamination G1 is similar to the gate lamination G of FIG. 1 but includes a nano-crystal array 20, instead of the single quantum dot 14.
The quantum dot 14 of the single electron memory device shown in FIG. 1 is formed using a nano-lithography technique. The nano-crystal array of the single electron memory device shown in FIG. 2 is formed utilizing a self-assembled growth method.
In the single electron memory devices, the thickness of the tunneling oxide layer 12 is the main factor determining the reliability of the device, retention time of information and speed of write/erase. The thickness of the control oxide layer 16 and the distribution density of the quantum dot are the main factors determining the extent of variation in threshold voltage.
In the conventional single electron memory devices, however, the quantum dot or the nano-crystal array is formed directly on the tunneling oxide layer 12, and thus, defects may occur in the tunneling oxide layer 12 while forming the quantum dot or the nano-crystal array. These defects may result in changes in the properties of the device thereby requiring many limitations in the formation of the quantum dot on the tunneling oxide layer 12.
For example, in order to implement single electron tunneling at room temperature when the quantum dot is formed of silicon, preferably, the size of the quantum dot is less than 10 nm. It is difficult, however, to form the quantum dot having a predetermined size due to the possibility of causing defects in the tunneling oxide layer. Hence, it becomes difficult to manufacture a single electron memory device, which operates at room temperature. Further, in the prior art single electron memory device, the retention time of information does not reach a practical standard.